Subject: [PATCH v3] netlink: Return unsigned value for nla_len()

From: Kees Cook <[email-masked]>
Date: Wed, 6 Dec 2023 12:59:07 -0800

The return value from nla_len() is never expected to be negative, and can
never be more than struct nlattr::nla_len (a u16). Adjust the prototype
on the function. This will let GCC's value range optimization passes
know that the return can never be negative, and can never be larger than
u16. As recently discussed[1], this silences the following warning in
GCC 12+:

net/wireless/nl80211.c: In function 'nl80211_set_cqm_rssi.isra':
net/wireless/nl80211.c:12892:17: warning: 'memcpy' specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
12892 |                 memcpy(cqm_config->rssi_thresholds, thresholds,
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12893 |                        flex_array_size(cqm_config, rssi_thresholds,
      |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12894 |                                        n_thresholds));
      |                                        ~~~~~~~~~~~~~~

A future change would be to clamp the subtraction to make sure it never
wraps around if nla_len is somehow less than NLA_HDRLEN, which would
have the additional benefit of being defensive in the face of nlattr
corruption or logic errors.

Reported-by: kernel test robot <[email-masked]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email-masked]/ [1]
Cc: Jakub Kicinski <[email-masked]>
Cc: David S. Miller <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: Johannes Berg <[email-masked]>
Cc: Jeff Johnson <[email-masked]>
Cc: Michael Walle <[email-masked]>
Cc: Max Schulze <[email-masked]>
Cc: [email-masked]
Cc: [email-masked]
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Kees Cook <[email-masked]>
---
 v3: do not cast NLA_HDRLEN to u16 (nicolas.dichtel)
 v2: https://lore.kernel.org/all/[email-masked]/
 v1: https://lore.kernel.org/all/[email-masked]/
---
 include/net/netlink.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 83bdf787aeee..7678a596a86b 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1200,7 +1200,7 @@ static inline void *nla_data(const struct nlattr *nla)
  * nla_len - length of payload
  * @nla: netlink attribute
  */
-static inline int nla_len(const struct nlattr *nla)
+static inline u16 nla_len(const struct nlattr *nla)
 {
 	return nla->nla_len - NLA_HDRLEN;
 }
-- 
2.34.1